PRACTICAL 11: CONVECTIve MASS TRANSFER
1. From the equation below, determine the units of film mass transfer coefficients kc, kg, ky, kx, kL:
NA”= NA/A=kc(CA1-CA2)= kg(pA1-pA2)= ky(yA1-yA2)= kx(xA1-xA2) = kL(CA1-CA2)
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2. A fluid with a stream velocity of 2 m/s is flowing over a flat plate of length 2 m. The flow conditions are at 298K and 1 atm. Make suitable assumptions to estimate the film convective mass transfer coefficient for:
(a) air flowing over a surface coated with water
(b) air flowing over a surface coated with naphthalene (where the DAB for naphthalene in air is 0.61 x 10-15 m2/s.
3. Consider a circular pipe of inner diameter 5 cm, whose inner surface is covered with a thin layer of liquid water as a result of condensation. In order to dry the pipe, air at 450K and 1 atm is forced to flow through the pipe with an average velocity of 1 m/s. Estimate the film convective mass transfer coefficient for evaporation of water from the pipe.
4. Ammonia is to be transferred from air into water using a “scrubbing” (i.e. solute transferred from gas phase to water phase) process in order to reduce ammonia emissions.
a) Calculate the kL NH3 (liquid film coefficient) for ammonia, if that of the oxygen in the water is 0.02 kmol m-2s-1 (kmol m-3)-1 using the equation below, if the diffusion coefficients for ammonia and oxygen in water under these conditions are 1.77 x 10-9 m2s-1 and 2.25 x 10-9 m2s-1 respectively.
kL NH3 a= kL NH3 a (D NH3 in H2O/ D O2 in H2O)0.5
where a is a measure of the interfacial area per unit volume of mass transfer
operation, and has the units m2 m-3
b) Calculate the gas film coefficient kg NH3 for ammonia, if the overall mass transfer coefficient for ammonia is 0.0008 kmol m-2 s-1 kPa-1 and the Henry’s law coefficient m=10
c) What will the units of m be?
d) Calculate the percentage of resistance to mass transfer in the liquid film
6. An interphase convective mass-transfer process involves the transfer of the industrial contaminant, methylene chloride (species A), between air and water at 20 oC and 2.20 atm total system pressure. Air is the inert carrier gas, and water is the inert solvent. The equilibrium mole fraction of methylene chloride dissolved in water vs the mole fraction of methylene chloride in air at 20 oC and 2.2 atm pressure is shown below.
At the present operating conditions, the bulk phase mole fraction of methylene chloride is 0.25 in the gas phase and 0.0020 in the liquid phase. The fluid flow rate associated with each case predicts a gas film mass transfer coefficient, ky, to be 0.010 gmol m-2 s-1 and a liquid film mass transfer coefficient , kx, to be 0.125 gmol m-2 s-1. At 20 oC, the density of liquid water is 992.3 kgm-3.
· a) Is the process a liquid stripping or a gas absorption process?
· b) Determine the Henry’s law constant H for methylene chloride dissolved in
water, according to the definition pA=HCA
· c) Determine the overall mass transfer coefficient KL
· d) What is the flux of species A across the gas and liquid phases?
· e) Determine the interface mole fractions xAi and yAi
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